There has been conventionally known a device that executes control of forcefully and periodically switching an exhaust air-fuel ratio at an upstream side from a three-way catalyst to a rich side and a lean side from stoichiometry (hereinafter, called “air-fuel ratio active control”), and detects degradation of the three-way catalyst. For example, Patent Literature 1 discloses the catalyst-degradation detection device that detects degradation of a three-way catalyst by using a stoichiometry holding time period of an exhaust air-fuel ratio that is detected by an air-fuel ratio sensor at a downstream side from the three-way catalyst during execution of air-fuel ratio active control.
The three-way catalyst has an ability to adsorb and store oxygen in exhaust emission when the exhaust air-fuel ration is lean, and to emit the oxygen that is adsorbed and stored when the exhaust air-fuel ratio is rich. Therefore, during execution of air-fuel ratio active control, the stoichiometry holding time period in which the exhaust air-fuel ratio at the downstream side from the three-way catalyst temporarily remains stoichiometric can be detected by the above described air-fuel ratio sensor. The stoichiometry holding time period has correlation with the amount of oxygen stored in or emitted from the three-way catalyst during this time period, and becomes short due to reduction in oxygen storage capacity (OSC) of the three-way catalyst. Accordingly, if the stoichiometry holding time period is used, degradation relating to the OSC of the three-way catalyst can be detected.